Organic electronic devices including circuits, for example, organic light emitting diodes, organic electrochromic displays, organic photovoltaic devices and organic thin film transistors, are known and are becoming increasingly important from an economic standpoint.
As a specific example, organic light emitting devices (“OLEDs”), including both polymer and small-molecule OLEDs, are potential candidates for a great variety of virtual- and direct-view type displays, such as lap-top computers, televisions, digital watches, telephones, pagers, cellular telephones, calculators and the like. Unlike inorganic semiconductor light emitting devices, organic light emitting devices are generally simple and are relatively easy and inexpensive to fabricate. Also, OLEDs readily lend themselves to applications requiring a wide variety of colors and to applications that concern large-area devices.
In general, two-dimensional OLED arrays for imaging applications are known in the art and typically include an OLED region, which contains a plurality of pixels arranged in rows and columns. FIG. 1A is a simplified schematic representation (cross-sectional view) of an OLED structure of the prior art. The OLED structure shown includes an OLED region 15 which includes a single pixel comprising an electrode region such as anode region 12, a light emitting region 14 over the anode region 12, and another electrode region such as cathode region 16 over the a light emitting region 14. The OLED region 15 is disposed on a substrate 10.
Traditionally, light from the light-emitting layer 14 is passed downward through the substrate 10. In such a “bottom-emitting” configuration, the substrate 10 and anode 12 are formed of transparent materials. The cathode 16 and cover 20 (i.e., barrier), on the other hand, need not be transparent in this configuration.
Other OLED architectures are also known in the art, including “top-emitting” OLEDs and transparent OLEDs. For top-emitting OLEDs, light from the light emitting layer 14 is transmitted upward through cover 20. Hence, the substrate 10 can be formed of opaque material, if desired, while the cover 20 is transparent. In top-emitting configurations based on a design like that illustrated in FIG. 1A, a transparent material is used for the cathode 16, while the anode 12 need not be transparent.
For transparent OLEDs, in which light is emitted out of both the top and bottom of the device, the substrate 10, anode 12, cathode 16 and cover 20 are all transparent.
Structures are also known in which the positions of the anode 12 and cathode 16 in FIG. 1A are reversed as illustrated in FIG. 1B. Such devices are sometimes referred to as “inverted OLEDs”.
In forming an OLED, a layer of low work function metal is typically utilized as the cathode to ensure efficient electron injection and low operating voltages. Low work function metals, however, are chemically reactive; exposure to and subsequent reaction with oxygen and moisture can severely limit the lifetime of the devices. Moisture and oxygen are also known to produce other deleterious effects, for instance, reactions with the organic materials themselves. For example, moisture and oxygen are known in the art to increase “dark spots” and pixel shrinkage in connection with OLEDs.
With the aid of a sealing region 25, the cover 20 and the substrate 10 cooperate to restrict transmission of oxygen and water vapor from an outer environment to the active pixel 15. Typically, the cover 20 is attached to the substrate 10 via sealing region 25 under a clean, dry, inert atmosphere. The cover is commonly made from glass, metal or plastic, with an indentation or cavity in the cover that provides a location for a getter material, which may be in the form of a pouch, thin film or thick film.
Sealing region 25 is commonly a ring of UV-curable liquid adhesive, such as an epoxy resin. Epoxy resins, however, are typically not flexible, rendering these materials undesirable for use in connection with flexible OLEDs. In addition, because they are typically inflexible, because they are not pressure sensitive, and because they are typically applied in liquid form, epoxy resins are not readily adaptable for use in web-based manufacturing techniques. Moreover, epoxy resins frequently contain ingredients that are deleterious to OLEDs. Analogous difficulties are encountered in organic electronic devices other than OLEDs.
Another type of adhesive material that is currently being utilized is a UV-curable pressure sensitive adhesive. This material is typically provided between two carrier films. Upon removal of one of the carrier films, the exposed adhesive, being pressure sensitive, is attached to either the cover or the substrate by simple contact. Subsequently, the second carrier film is removed, allowing the cover and the substrate to be attached to one another. Curing is completed by the application of ultraviolet-radiation.
Although such adhesive layers do provide a degree of protection from the outside environment, the barrier properties of these adhesive layers are often insufficient to protect the OLED device from premature degradation in commercial applications.